1. Field of the Invention
The present invention relates to a liquid crystal lens assembly, more particularly, to a two dimension/three dimension (2D/3D) switchable liquid crystal lens assembly having high lens power.
2. Description of the Related Art
Real-world images are perceived by the human eyes, and further, so-called three-dimensional (3D) images are perceived by the human brain depending on an apparent displacement of an object viewed along two different lines of sight. Such a displacement or a spatial difference is called parallax. A so-called 3D display device, simulating human vision to form different viewing angles, is capable of conveying 3D images to the viewer. The 3D display device produces two different 2D images with parallax, one for the viewer's right eye and the other for the left eye. Afterwards, the viewer's brain perceives these two different 2D images as a 3D image.
Nowadays, there are two types of 3D display devices in general, auto-stereoscopic displays and stereoscopic displays. A user of an auto-stereoscopic display can see 3D images without wearing special type glasses. As for a user of a stereoscopic display, he/she has to wear special type glasses to see 3D images. A commonly seen auto-stereoscopic display is divided into two types: parallax barrier auto-stereoscopic displays and lenticular lenses auto-stereoscopic displays. The theorem of parallax barrier auto-stereoscopic displays lies on that a user sees parallax images through both eyes by controlling light propagation direction with an opaque parallax barrier, and the parallax images is perceived as stereoscopic view in the brain.
As for lenticular lenses auto-stereoscopic displays, light propagation direction is controlled by varying the refractive index, which can be realized by different methods. One of them is to make a liquid crystal layer function as a physical lens. With the specially patterned indium tin oxide electrodes on top and bottom substrates, the unevenly distributed electric field lines are generated. The alignment of the liquid crystal molecules is thus changed to result in their different refractive indexes. Hence, the whole liquid crystal layer will behave like a lenticular lens to control the refraction direction of the incident light if a proper design is implemented.
Please refer to FIG. 1a and FIG. 1b, FIG. 1a is a schematic diagram showing a conventional GRIN lens without being applied with a voltage according to the prior art. FIG. 1b is a schematic diagram showing the conventional GRIN lens being applied with a voltage. The GRIN lens (gradient index lens) is characterized by its refractive index distribution that varies with the distance to the axis. When no voltage is applied on the GRID lens, liquid crystal molecules will align in a manner as shown in FIG. 1a. Owing to the existence of the above-mentioned indium tin oxide electrode pattern (not indicated), the liquid crystal molecules will align in another manner, as shown in FIG. 1b, when a voltage is applied on the GRIN lens. The electric field to be generated will cause the liquid crystal molecules at the center of the lens have the highest refractive index (ne), and decreases gradually from the center to both edges of the lens until the lowest refractive index (no). When light propagates into the GRIN lens, the light travels through both edges of the lens will have a highest speed because of the lowest refractive index of the liquid crystal molecules near the edges. While the light travels through the center of the lens will have a lowest speed because of the highest refractive index of the liquid crystal molecules near the center. For an incident plane wave, the wavefront is thus curved. The curved wavefront, similar to a convex lens, is what focus the light on a focus point F. The equation for focal length is derived as follows:
            f      GRIN        =                  r        2                    2        ⁢                  d          ⁡                      [                                          n                max                            -                              n                ⁡                                  (                  r                  )                                                      ]                                ,where ‘fGRIN’ is the focal length of the GRIN lens 10; “d” is the cell thickness; “r” is the radius of the GRIN lens 10; nmax is the extraordinary refractive index ne of the liquid crystal molecules; “n(r)” means that refractive index is a function of r. When a design goal of 4 mm focal length is implanted, the cell thickness d must be maintained at approximately 30 μm if the refractive index difference Δn is 0.21. Therefore, in order to realize a small focal length, the only choice is to increase the cell thickness if the radius of the GRIN lens 10 cannot be reduced and the type of the liquid crystal molecules cannot be changed. However, the increased cell thickness not only increases the cell gap, but also increases the cost. It is therefore very important to provide a 2D/3D switchable liquid crystal lens assembly to improve the focus ability without increasing the cost resulted from the increased cell thickness.